Origin of the Moon

The origin of the Moon has been endlessly debated, but the mystery still has not been ultimately solved. There are three classical theories; the coaccretion model, the fission model and the intact capture model.

The coaccretion model assumes that Moon and Earth developed close to each other as separate bodies. This model is based on the model for the origin of the solar system, suggesting that it condensed out of a cloud of hot gas, called the solar nebula. In such a model the planets are small pieces of the condensed material left over after star formation. Thus, our Moon and the moons of the other planets would be similar to the planets that form around a star, a moon being the left-over bits of a planet formation. One of the major problems of this model is the fact that the densities of Moon and Earth are very different. This appears unlikely if they were formed at the same time, in the same region and from the same original matter. Also, there is a problem in accounting for the angular momentum of the Earth and Moon to explain why material around Earth stayed in orbit rather than falling on Earth.

The fission model assumes that the Moon was initially a part of Earth. This theory is based upon the assumption that initially the original planet had a high rotational rate and led to the suggestion that the Moon was flung off Earth very early in Earth's history. According to calculations, this offspring should have a mass of about 1–2% of the original body's mass, and the ratio of the diameters of the resulting spherical bodies should be about 1:4. These are quite exactly the ratios of Earth and Moon. However, there are problems to explain how the Earth was spinning fast enough to fling off a blob of material from which the Moon formed. Because no explanation for this aspect could be found, the fission model had to be rejected.

The intact capture model assumes that the Moon's and Earth's flight paths crossed early in the history of the solar system, resulting in the Moon being captured by Earth. But, according to this model, the minimum distance of Earth and Moon would have to be less than 60,000 km. Also, the fact that Moon's orbit around Earth is almost circular and the symmetry of the Earth–Moon system make this theory appear very unlikely. Moreover, the probability of capturing a fully formed Moon is considered extremely tiny.

Thus, none of the three classical models of lunar origin are particularly successful at explaining all the properties of the Moon and all the orbital characteristics of the Earth–Moon system. Therefore, the giant impact (or Big Whack) model was created, which combines essential aspects of the three classical models. The idea that the Moon formed as a consequence of a giant impact of a planetoid on Earth, originally proposed in 1976, is currently accepted wisdom in planetary science. It assumes that during the last phase of Earth's accretion, after its core had formed and while it was still molten, an object the size of Mars smashed into this proto-Earth at an oblique angle (capture model). The resulting monumental explosion deposited large quantities of material in Earth orbit, and the primitive Moon formed from this material (fission model).

Although the giant impact model can explain most of the current phenomena of the Earth–Moon system, including the geochemical properties of the Moon and the orbital dynamics of the Earth–Moon system (coaccretion model), it is far from certain that the problem of lunar origin has been solved and more data will be required either to prove or to reject it.